In response to your items listed above:

1. By all means put in seat belts (shoulder straps can also be installed on both sides, unless you have a panel body truck).

2. Unless you need a lot of 12v high-power devices, 6v volt works reliably.

- have a good 6v battery

- make sure both the battery cables and all the grounding straps (usually braided) are 2 gauge or 1 gauge, or 0 gauge, or 00 gauge
(size matters - 2 gauge was original on 6v and 4 gauge was original in 12v)

- check/clean ALL grounding locations - poor grounds are the main problem causing complaints about 6v systems

3. Inexpensive 6v-12v converters are available, if you want to use a few low-power-draw 12v devices (like GPS or cell phone chargers)

I have had only one problem with 6v on my 1954 Suburban that I have owned/driven for 47 years.
That one problem was a combination of having a combination of 12v cable and a bad ground.